Laser welding of plastic is an important emerging industrial process that utilizes near-infrared (NIR) lasers to weld pieces of plastic together.
For use in industrial laser welding processes NIR pigments such as Carbon black, various metallated phthalocyanines and more recently differently substituted perylenes have been used. These classes of compounds are well-studied, chemically inert, heat resistant, and often relatively cheap to produce. However, they suffer from the disadvantage, that they are colored in the visible region, thus making them less ideal for the use in laser welding of colorless plastics. An issue with most of these dyes is their chemical stability; the stability leaves the plastic colored after the welding has been performed.
Specific examples of prior art publications include WO 2009/024497 disclosing a composition comprising titanium dioxide in the anatase form and a polymeric binder. The composition may also comprise tungsten suboxide, tungsten bronze, or mixtures of tungsten trioxide, tungsten bronze and metallic tungsten. The composition is used for forming a laser-sensitive coating layer on a substrate.
WO2009/059900 relates to the use of tungsten oxide or of tungstate to increase the heat-input amount of near infrared radiation in laser welding of plastics. Tungsten oxide to be using in the invention has the formula W03-x wherein W is tungsten, O is oxygen, and x is 0.1-1 and/or the use of tungstate of the formula MxWyOz wherein M is one or more element selected from NH4, H, Li, Na, K, Rb, Cs, Ca, Ba, Sr, Fe, Sn, Mo, Nb, Ta, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Tl, W is tungsten, O is oxygen, 0.001<x/y<1, and 2.0<z/y<3.0, to increase the heat-input amount of near infrared radiation in the process of NIR curing of coatings and NIR drying of coatings.
WO 2010/046285 also relates to welding of plastic using a tungsten derivative. WO 2011/005631 relates to potassium cesium tungsten bronze solid solution particles, which are indicated to be NIR absorbers. The particles may be included in plastics.
WO 2009/010405 also discloses a laser-sensitive system. Preferably, the laser-sensitive system is selected from the group consisting of a salt of an acid and an amine or mixtures of salts of acids and amines, titanium dioxide, an oxygen containing transition metal salt, a compound containing a free carbonyl group and a nucleophile or a compound containing a free carbonyl group, which compound is substituted with one or more nucleophilic groups, a compound having functional groups and a metal compound or an acid, and a color former and a color developer or a latent color developer which generates a color developer upon activation, preferably a color former and a latent color developer.
WO 2007/114829 discloses a coating composition comprising a mark formation layer, which contains an electron donor dye precursor micro-encapsulated within a polymer of a specific Tg range, and an electron acceptor compound which can react with the electron donor dye to turn it into a dye.
Dyes that are used as laser absorber dyes should fulfill certain requirements. The dye must have a strong absorption at the wavelength of the used laser, it should be sufficiently heat resistant to resist the treatment during compounding and neither the dye nor the decomposition products should be toxic if used together with foodstuff etc.
The present invention provides laser absorbing dyes, which in addition to the above requirements, also are almost colorless in the visible area. A colorless laser absorbing dye may provide for almost invisible sealing of transparent plastic elements to each other. In a preferred embodiment the laser absorbing dyes for plastic laser welding is colorless in the visible region and absorbs intensely in the NIR region, where the laser of choice emits. Furthermore, the present invention provides in a preferred embodiment that the NIR absorption of the dye is transmitted to the surroundings in the form of heat, thus causing the welding to occur. The NIR absorber of the present invention for laser welding of plastic does not, or only to a limited extent, releases its energy via other pathways than heat, such as fluorescence.